1. Field of the Invention
The present invention relates to a solid polyelectrolyte membrane for fuel cells, and to a method for producing it.
2. Discussion of the Background
A polyelectrolyte-type fuel cell comprises a gas-diffusing cathode electrode, a gas-diffusing anode electrode and a solid polyelectrolyte membrane. Oxygen gas and hydrogen gas are supplied to the cathode and the anode, respectively, to generate electricity between the electrodes. The function of the polyelectrolyte membrane is to transport hydrogen ions, formed around the gas-diffusing anode electrode, to the gas-diffusing cathode electrode. Around the gas-diffusing cathode electrode, the thus-transported hydrogen ions react with oxygen gas and electrons to produce water.
The performance of the cell depends on the catalytic activity of the catalyst used in each gas-diffusing electrode, the gas-diffusing ability of each electrode, the hydrogen ion conductivity of the solid polyelectrolyte membrane, etc. Therefore, the internal resistance of the cell could be smaller when the solid polyelectrolyte membrane in the cell has a higher hydrogen ion conductivity, resulting in superior performance of the cell.
The solid polyelectrolyte membrane is produced by forming a resin having an ion-exchanging function into films; a membrane having a higher degree of ion-exchanging capacity having a higher degree of ion conductivity. Compared with other cells, the solid polyelectrolyte-type fuel cell is more compact and produces more power. Therefore, in the future, this type of fuel cell will be widely used as the power source for electric cars.
In solid polyelectrolyte-type fuel cells produced in the early years, an ion-exchange membrane used was prepared by infiltrating monomers of styrene and divinylbenzene into a reinforcing cloth followed by copolymeriztion, as the electrolyte membrane. However, since its durability is extremely poor, this ion-exchange membrane is impractical. Since then, perfluorosulfonic acid membranes (trade name, NAFION) developed by DUPONT have been generally used.
The perfluorosulfonic acid membranes have good ion conductivity and durability. However, since they are formed from a fluorine resin, they are extremely expensive, which is a serious bar to the commercialization of solid polyelectrolyte-type fuel cells for electric cars. For these reasons, various studies have heretofore been made for developing inexpensive electrolyte membranes capable of being substituted for NAFION. However, it has been presumed that electrolyte membranes essentially comprising a hydrocarbon resin will decompose at the hydrocarbon polymer chains by the action of peroxides or active radicals formed as intermediates in the electrode reaction, and it has been reported that, when such electrolyte membranes are used in cells, the cell output is lowered immediately after the cells have started.